1. Field of Invention
The present invention relates to techniques for determining a scale build-up in pipes, tanks, vessels or containers; and more particular to techniques for determination of a scale build-up in pipes, tanks, vessels or containers using tomographic techniques.
2. Description of Related Art
Tomographic techniques or approaches based on the use of Electrical Resistance Tomography (ERT), Electrical Capacitance Tomography (ECT) and Electrical Impedance Tomography (EIT) are becoming widely exploited in industrial processes for the analysis of mixing in multi-phase flows, liquid interfaces and liquid-froth layers for example.
These techniques or approaches are based at least partly on the difference in conductivity or electrical (complex) permeability of materials or mediums under investigation. In such a technique, the aim is to image the cross-section of the fluid in, e.g., a pipe, to indicate:
1) The mixing of different fluidic components,
2) Air content, and/or
3) The degree/content of solids in the flow.
By way of example, the use of tomographic analysis using linear-geometry probe sensors has also been disclosed and is known in the art. In such an application, the fluid composition in the volume surrounding the probe (the measurement volume) can be visualized using tomographic processing. This type of probe has been applied to measurements taken in flotation cells, e.g., in mineral separation processes. In such an application, as with many others, scale build-up on sensor electrodes can give an erroneous tomographic image of the measurement volume.
Scale build-up on electrodes results in a layer of high resistivity material on the electrode, known as the electrode-fluid interface conductivity, as shown in FIG. 1. This layer increases in resistance (drops in conductivity) as the scale deposits build-up. By way of example, FIGS. 1b, 1c and 1d respectively show such a pipe having substantially zero scale build-up, low scale build-up, and high scale build-up.
In order to create tomographic images, it is known to use, e.g., an in-pipe tomography system as shown in FIG. 1a, and to space a set of electrodes in regular intervals around the circumference of the pipe, e.g., in a so-called regular configuration. See also that shown in FIG. 4a. The regular configuration may include, or take the form of, evenly or symmetrically spaced electrodes in regular intervals and/or electrodes having substantially the same width or length. In operation, current is sent between a pair of electrodes and the potentials generated across other pairs are monitored. When the tomographic electrode array is first deployed, the impedance between two adjacent electrodes varies with the conductivity of the fluid in the measurement volume. For a pipe-based array as shown in FIG. 1a, the conductivity of the fluid, Kf, fluctuates with the variability in the process fluid. In most normal or typical applications, the range in conductivity (ΔKf) of the process fluid is bounded by the constituents of fluid (in the case of mixed fluids), or the degree of dissolved or suspended particles in the flow. As an example, see the graph in FIG. 2, which shows process fluid conductivity in relation to time and the variation in the conductivity of a process fluid.
This range of variance in conductivity, ΔKf, occurs at fluid mixing/process variability time frames, typically in sub-second to minute time frames. As scale builds up on the electrodes, the electrode-fluid interface conductivity (Ke) drops the measured conductivity, K, of the fluid, consistent with the relationship set forth in the equation, as follows:K=Kf*Ke/(Kf+Ke).
The measured conductivity, K, typically appears to fall over time, consistent with that set forth in the graph in FIG. 3, which shows apparent process fluid conductivity in relation to time and the drop in apparent conductivity with increasing scale, while the fluidic time-varying component remains, but is now suppressed in amplitude due to the scale build-up.
Tomographic processing algorithms are known in the art and have been developed to compensate for this scale build-up by compensating for the electrode-fluid interface conductivity, using a model that slow increments of an inputted value of the effective electrode-fluid interface conductivity Kf over time to ‘restore’ the correct variability in the fluidic conductivity. As the scale builds up over time in an incrementing fashion, and initiates on ‘clean’ electrodes, some boundaries are set for electrode-fluid interface conductivity Ke, as follows:e.g.: at t=0, Ke=0,andat t2>t1, Ke(t2)<Ke(t1).
In this way, the electrode-fluid interface conductivity Ke can be inferred, and the compensation applied to a measure of the fluid conduction (and this composition).
In most industrial processes, the potential for scale build-up is known, as well as the types of deposits (e.g., calcium carbonate) that are building up.
In view of the aforementioned understanding, there is a need in the industry for a different and better way to determine the scale build-up, e.g., in a pipe, tank, cell or vessel.